Syndecans in cell adhesion and differentiation

Syndecans are transmembrane proteoglycans expressed on adherent cells. They are a family of four proteins, which participate in cell-matrix adhesion, the regulation of growth factors (FGFs, VEGF, HGF) binding and signaling. The extracellular domain of syndecans contains heparan sulfate and chondroitin sulfate glycosaminoglycan chains. Syndecans have transmembrane region and a short cytoplasmic domain. The cytoplasmic domain attaches activated protein kinase Calpha, phosphatidyl-inositol-4,5-bisphosphate, syntenin, beta-catenin and many others molecules. Syndecans bind numerous ligands, which are present in extracellular matrix: growth factors, enzymes, extracellular matrix molecules (fibronectin, laminin). They form connections with actin cytoskeleton. The changes in syndecan expression influence on cell adhesion and migration, structure of focal contacts and cytoskeleton. Syndecans participate in cell differentiation and tissue regeneration.     

Syntenin-syndecan binding requires syndecan-synteny and the co-operation of both PDZ domains of syntenin

Syntenin is an adaptor-like molecule that binds to the cytoplasmic domains of all four vertebrate syndecans. Syntenin-syndecan binding involves the C-terminal part of syntenin that contains a tandem of PDZ domains. Here we provide evidence that each PDZ domain of syntenin can interact with a syndecan. Isolated or combined mutations of the carboxylate binding lysines in the inter-betaAbetaB loops and of the alphaB1 residues in either one or both the PDZ domains of syntenin all reduce syntenin-syndecan binding in yeast two-hybrid, blot-overlay, and surface plasmon resonance assays. PDZ2 mutations have more pronounced effects on binding than PDZ1 mutations, but complete abrogation of syntenin-syndecan binding requires the combination of both the lysine and the alphaB1 mutations in both the PDZ domains of syntenin. Isothermal calorimetric titration of syntenin with syndecan peptide reveals the presence of two binding sites in syntenin. Yet, unlike a tandem of two PDZ2 domains and a reconstituted PDZ1+PDZ2 tandem, a tandem of two PDZ1 domains and isolated PDZ1 or PDZ2 domains do not interact with syndecan bait. We conclude to a co-operative binding mode whereby neither of these two PDZ domains is sufficient by itself but where PDZ2 functions as a "major" or "high affinity" syndecan binding domain, and PDZ1 functions as an "accessory" or "low affinity" syndecan binding domain. The paired, but not the isolated PDZ domains of syntenin bind also strongly to the immobilized cytoplasmic domains of neurexin and B-class ephrins. By inference, these data suggest a model whereby recruitment of syntenin to membrane surfaces requires two compatible types of bait that are in "synteny" (occurring together in location) and engages both PDZ domains of syntenin. The synteny of compatible bait may result from the assemblies and co-assemblies of syndecans and other similarly suited partners in larger supramolecular complexes. In general, an intramolecular combination of PDZ domains that are weak, taken individually, would appear to be designed to detect rather than drive the formation of specific molecular assemblies.     

Syndecans promote integrin-mediated adhesion of mesenchymal cells in two distinct pathways

Syndecans are transmembrane proteoglycans that support integrin-mediated adhesion. Well documented is the contribution of syndecan-4 that interacts through its heparan sulphate chains to promote focal adhesion formation in response to fibronectin domains. This process has requirements for integrin and signaling through the cytoplasmic domain of syndecan-4. Here an alternate pathway mediated by the extracellular domains of syndecans-2 and -4 is characterized that is independent of both heparan sulphate and syndecan signaling. This pathway is restricted to mesenchymal cells and was not seen in any epithelial cell line tested, apart from vascular endothelia. The syndecan ectodomains coated as substrates promoted integrin-dependent attachment, spreading and focal adhesion formation. Syndecan-4 null cells were competent, as were fibroblasts compromised in heparan sulphate synthesis that were unable to form focal adhesions in response to fibronectin. Consistent with actin cytoskeleton organization, the process required Rho-GTP and Rho kinase. While syndecan-2 and -4 ectodomains could both promote integrin-mediated adhesion, their pathways were distinct, as shown by competition assays. Evidence for an indirect interaction of beta1 integrin with both syndecan ectodomains was obtained, all of which suggests a distinct mechanism of integrin-mediated adhesion.     

Syndecans in wound healing, inflammation and vascular biology

Syndecans are heparan sulphate proteoglycans consisting of a type I transmembrane core protein modified by heparan sulphate and sometimes chondroitin sulphate chains. They are major proteoglycans of many organs including the vasculature, along with glypicans and matrix proteoglycans. Heparan sulphate chains have potential to interact with a wide array of ligands, including many growth factors, cytokines, chemokines and extracellular matrix molecules relevant to growth regulation in vascular repair, hypoxia, angiogenesis and immune cell function. This is consistent with the phenotypes of syndecan knock-out mice, which while viable and fertile, show deficits in tissue repair. Furthermore, there are potentially important changes in syndecan distribution and function described in a variety of human vascular diseases. The purpose of this review is to describe syndecan structure and function, consider the role of syndecan core proteins in transmembrane signalling and also their roles as co-receptors with other major classes of cell surface molecules. Current debates include potential redundancy between syndecan family members, the significance of multiple heparan sulphate interactions, regulation of the cytoskeleton and cell behaviour and the switch between promoter and inhibitor of important cell functions, resulting from protease-mediated shedding of syndecan ectodomains.     

Syndecans as cell surface receptors: Unique structure equates with functional diversity

An increasing number of functions for syndecan cell surface heparan sulfate proteoglycans have been proposed over the last decade. Moreover, aberrant syndecan regulation has been found to play a critical role in multiple pathologies, including cancers, as well as wound healing and inflammation. As receptors, they have much in common with other molecules on the cell surface. Syndecans are type I transmembrane molecules with cytoplasmic domains that link to the actin cytoskeleton and can interact with a number of regulators. However, they are also highly complex by virtue of their external glycosaminoglycan chains, especially heparan sulfate. This heterodisperse polysaccharide has the potential to interact with many ligands from diverse protein families. Here, we relate the structural features of syndecans to some of their known functions.     

Molecular and cellular mechanisms of syndecans in tissue injury and inflammation

The syndecan family of heparan sulfate proteoglycans is expressed on the surface of all adherent cells. Syndecans interact with a wide variety of molecules, including growth factors, cytokines, proteinases, adhesion receptors and extracellular matrix components, through their heparan sulfate chains. Recent studies indicate that these interactions not only regulate key events in development and homeostasis, but also key mechanisms of the host inflammatory response. This review will focus on the molecular and cellular aspects of how syndecans modulate tissue injury and inflammation, and how syndecans affect the outcome of inflammatory diseases in vivo.     

Syndecans as receptors and organizers of the extracellular matrix

Syndecans are type I transmembrane proteins having a core protein modified with glycosaminoglycan chains, most commonly heparan sulphate. They are an ancient group of molecules, present in invertebrates and vertebrates. Among the plethora of molecules that can interact with heparan sulphate, the collagens and glycoproteins of the extracellular matrix are prominent. Frequently, they do so in conjunction with other receptors, most notably the integrins. For this reason, they are often referred to as “co-receptors”. However, just as with integrins, syndecans can interact with actin-associated proteins and signalling molecules, such as protein kinases. Some aspects of syndecan signalling are understood but much remains to be learned. The functions of syndecans in regulating cell adhesion and extracellular matrix assembly are described here. Evidence from null mice suggests that syndecans have roles in postnatal tissue repair, inflammation and tumour progression. Developmental deficits in lower vertebrates in which syndecans are eliminated are also informative and suggest that, in mammals, redundancy is a key issue.     

A model for non-viral gene delivery: through syndecan adhesion molecules and powered by actin

BACKGROUND: Cell transfection requires cationic DNA complexes and heparan sulfate proteoglycans (HSPGs) at the cell surface. Syndecans are transmembrane HSPGs that are ubiquitously expressed on adherent cells. Their polyanionic heparan sulfate moieties are bound at the distal end of their ectodomain, thus facilitating interaction with large cationic particles. METHODS: We propose a model for cell entry involving syndecans as receptors for the DNA complexes by comparing transfection with bacteria uptake and using drug inhibition experiments along with confocal microscopy. RESULTS: When combined with results from the literature, our data suggest the following sequence of events: after initial particle binding, gradual electrostatic zippering of the plasma membrane onto the particle is sustained by lateral diffusion of syndecan molecules that cluster into cholesterol-rich rafts. Clustering in turn triggers PKC activity and linker protein-mediated actin binding to the cytoplasmic tail of the syndecans. Resulting tension fibers and a growing network of cortical actin may then pull the particle into the cell. CONCLUSIONS: Diversion of integrin- and syndecan-mediated cell adhesion processes for particle engulfment appears to be widely exploited by animals (chylomicrons), by pathogens (bacteria, viruses) and, as suggested here, by non-viral vectors.     

Characterization of syntenin, a syndecan-binding PDZ protein, as a component of cell adhesion sites and microfilaments

Syntenin is a PDZ protein that binds the cytoplasmic C-terminal FYA motif of the syndecans. Syntenin is widely expressed. In cell fractionation experiments, syntenin partitions between the cytosol and microsomes. Immunofluorescence microscopy localizes endogenous and epitope-tagged syntenin to cell adhesion sites, microfilaments, and the nucleus. Syntenin is composed of at least three domains. Both PDZ domains of syntenin are necessary to target reporter tags to the plasma membrane. The addition of a segment of 10 amino acids from the N-terminal domain of syntenin to these PDZ domains increases the localization of the tags to stress fibers and induces the formation of long, branching plasma membrane extensions. The addition of the complete N-terminal region, in contrast, reduces the localization of the tags to plasma membrane/adhesion sites and stress fibers, and reduces the morphotypical effects. Recombinant domains of syntenin with the highest plasma membrane localization display the lowest nuclear localization. Syndecan-1, E-cadherin, beta-catenin, and alpha-catenin colocalize with syntenin at cell-cell contacts in epithelial cells, and coimmunoprecipitate with syntenin from extracts of these cells. These results suggest a role for syntenin in the composition of adherens junctions and the regulation of plasma membrane dynamics, and imply a potential role for syntenin in nuclear processes.     

Human CASK/LIN-2 Binds Syndecan-2 and Protein 4.1 and Localizes to the Basolateral Membrane of Epithelial Cells

In Caenorhabditis elegans, mutations in the lin-2 gene inactivate the LET-23 receptor tyrosine kinase/Ras/MAP kinase pathway required for vulval cell differentiation. One function of LIN-2 is to localize LET-23 to the basal membrane domain of vulval precursor cells. LIN-2 belongs to the membrane-associated guanylate kinase family of proteins. We have cloned and characterized the human homolog of LIN-2, termed hCASK, and Northern and Western blot analyses reveal that it is ubiquitously expressed. Indirect immunofluorescence localizes CASK to distinct lateral and/or basal plasma membrane domains in different epithelial cell types. We detect in a yeast two-hybrid screen that the PDZ domain of hCASK binds to the heparan sulfate proteoglycan syndecan-2. This interaction is confirmed using in vitro binding assays and immunofluorescent colocalization. Furthermore, we demonstrate that hCASK binds the actin-binding protein 4.1. Syndecans are known to bind extracellular matrix, and to form coreceptor complexes with receptor tyrosine kinases. We speculate that CASK mediates a link between the extracellular matrix and the actin cytoskeleton via its interaction with syndecan and with protein 4.1. Like other membrane-associated guanylate kinases, its multidomain structure enables it to act as a scaffold at the membrane, potentially recruiting multiple proteins and coordinating signal transduction.     

The binding of the PDZ tandem of syntenin to target proteins

PDZ domains are among the most abundant protein modules in the known genomes. Their main function is to provide scaffolds for membrane-associated protein complexes by binding to the cytosolic, C-terminal fragments of receptors, channels, and other integral membrane proteins. Here, using both heteronuclear NMR and single crystal X-ray diffraction, we show how peptides with different sequences, including those corresponding to the C-termini of syndecan, neurexin, and ephrin B, can simultaneously bind to both PDZ domains of the scaffolding protein syntenin. The PDZ2 domain binds these peptides in the canonical fashion, and an induced fit mechanism allows for the accommodation of a range of side chains in the P(0) and P(-)(2) positions. However, binding to the PDZ1 domain requires that the target peptide assume a noncanonical conformation. These data help explain how syntenin, and perhaps other PDZ-containing proteins, may preferentially bind to dimeric and clustered targets, and provide a mechanistic explanation for the previously reported cooperative ligand binding by syntenin's two PDZ domains.     

The tandem PDZ domains of syntenin promote cell invasion

Syntenin is a tandem PDZ protein that has recently been shown to be overexpressed in several cancer cells and tissues, and that might play an active role in tumor cell invasion and metastasis. Here we show that overexpression of the tandem PDZ domains of syntenin in non-invasive cells is necessary and sufficient to stimulate these cells to invade a collagen I matrix, and this effect can be regulated by ligand binding to the PDZ domains. Furthermore, we show that syntenin-induced invasion requires signaling through ras, rho and PI3K/MAPK signaling pathways and involves changes in cell-cell adhesion. Inversely, when we used RNA interference to inhibit syntenin expression in different invasive cancer cell lines, we observed a drastically decreased ability of these cells to migrate and invade into collagen type I or Matrigel. RNAi-treated cells also show increased cell aggregation, indicating that syntenin is important for cell-cell adhesion in epithelial cells. Together, these results suggest that downregulation of syntenin by RNA interference could provide a means of inhibiting tumor invasion and possibly metastasis in different cancers, and point to syntenin as a potential cancer biomarker and drug target.     

Molecular roots of degenerate specificity in syntenin's PDZ2 domain: reassessment of the PDZ recognition paradigm

Crystal structures of the PDZ2 domain of the scaffolding protein syntenin, both unbound and in complexes with peptides derived from C termini of IL5 receptor (alpha chain) and syndecan, reveal the molecular roots of syntenin's degenerate specificity. Three distinct binding sites (S(0), S(-1), and S(-2)), with affinities for hydrophobic side chains, function in a combinatorial way: S(-1) and S(-2) act together to bind syndecan, while S(0) and S(-1) are involved in the binding of IL5Ralpha. Neither mode of interaction is consistent with the prior classification scheme, which defined the IL5Ralpha interaction as class I (-S/T-X-phi) and the syndecan interaction as class II (-phi-X-phi). These results, in conjunction with other emerging structural data on PDZ domains, call for a revision of their classification and of the existing model of their mechanism.     

Heparanase activates the syndecan-syntenin-ALIX exosome pathway

Exosomes are secreted vesicles of endosomal origin involved in signaling processes. We recently showed that the syndecan heparan sulfate proteoglycans control the biogenesis of exosomes through their interaction with syntenin-1 and the endosomal-sorting complex required for transport accessory component ALIX. Here we investigated the role of heparanase, the only mammalian enzyme able to cleave heparan sulfate internally, in the syndecan-syntenin-ALIX exosome biogenesis pathway. We show that heparanase stimulates the exosomal secretion of syntenin-1, syndecan and certain other exosomal cargo, such as CD63, in a concentration-dependent manner. In contrast, exosomal CD9, CD81 and flotillin-1 are not affected. Conversely, reduction of endogenous heparanase reduces the secretion of syntenin-1-containing exosomes. The ability of heparanase to stimulate exosome production depends on syntenin-1 and ALIX. Syndecans, but not glypicans, support exosome biogenesis in heparanase-exposed cells. Finally, heparanase stimulates intraluminal budding of syndecan and syntenin-1 in endosomes, depending on the syntenin-ALIX interaction. Taken together, our findings identify heparanase as a modulator of the syndecan-syntenin-ALIX pathway, fostering endosomal membrane budding and the biogenesis of exosomes by trimming the heparan sulfate chains on syndecans. In addition, our data suggest that this mechanism controls the selection of specific cargo to exosomes.     

Syndecan 4 heparan sulfate proteoglycan is a selectively enriched and widespread focal adhesion component.

Focal adhesion formation in fibroblasts results from complex transmembrane signaling processes initiated by extracellular matrix molecules. Although a role for integrins with attendant tyrosine kinases has been established, there is evidence that cell surface heparan sulfate proteoglycans (HSPGs) are also involved with an associated role of protein kinase C. The identity of the proteoglycan has remained elusive, but we now report that syndecan 4 (ryudocan/amphiglycan) is present in focal adhesions of a number of cell types. Affinity-purified antibodies raised against a unique portion of the cytoplasmic domain of syndecan 4 core protein recognized an HSPG of similar characteristics to those of syndecan 4. These antibodies stained focal adhesions only after cell permeabilization and recognized differing mammalian species. Syndecan 4 was associated with focal adhesions that contained either beta 1 or beta 3 integrin subunits and those that formed on substrates of fibronectin, laminin, vitronectin, or type I collagen. No focal adhesions were found that were vinculin-containing but lacked syndecan 4. In contrast, syndecan 2, whose cytoplasmic domain is closely homologous to syndecan 4, does not appear to be a focal adhesion component. Thus, syndecan 4 represents a new transmembrane focal adhesion component, probably involved in their assembly.     

Syntenin and syndecan in the biogenesis of exosomes

Cells communicate with their environment in various ways, including by secreting vesicles. Secreted vesicles are loaded with proteins, lipids and RNAs that compose ‘a signature’ of the cell of origin and potentially can reprogram recipient cells. Secreted vesicles recently gained in interest for medicine. They represent potential sources of biomarkers that can be collected from body fluids and, by disseminating pathogenic proteins, might also participate in systemic diseases like cancer, atherosclerosis and neurodegeneration. The mechanisms controlling the biogenesis and the uptake of secreted vesicles are poorly understood. Some of these vesicles originate from endosomes and are called ‘exosomes’. In this review, we recapitulate recent insight on the role of the syndecan (SDC) heparan sulphate proteoglycans, the small intracellular adaptor syntenin and associated regulators in the biogenesis and loading of exosomes with cargo. SDC–syntenin‐associated regulators include the endosomal sorting complex required for transport accessory component ALG‐2‐interacting protein X, the small GTPase adenosine 5′‐diphosphate‐ribosylation factor 6, the lipid‐modifying enzyme phospholipase D2 and the endoglycosidase heparanase. All these molecules appear to support the budding of SDC–syntenin and associated cargo into the lumen of endosomes. This highlights a major mechanism for the formation of intraluminal vesicles that will be released as exosomes.     

Syndecans,signaling, and cell adhesion

Syndecans are transmembrane proteoglycans which can participate in diverse cell surface interactions, involving extracellular matrix macromolecules, growth factors, protease inhibitors, and even viral entry. Currently, all extracellular interactions are believed to be mediated by distinct structures within the heparan sulfate chains, leaving the roles of chondroitin sulfate chains and extracellular portion of the core proteins to be elucidated. Evidence that syndecans are a class of receptor involved in cell adhesion is mounting, and their small cytoplasmic domains may link with the microfilament cytoskeleton, thereby mediating signaling events. The molecular details are unknown, but the conservation of regions of syndecan cytoplasmic domains, and a strong tendency for homotypic association, support the idea that the ligand-induced clustering may be a discrete source of specific transmembrane signaling from matrix to cytoskeleton, as proposed for other classes of adhesion receptors. © 1996 Wiley-Liss, Inc.     

Syndecan-1-mediated cell spreading requires signaling by alphavbeta3 integrins in human breast carcinoma cells

Syndecans are cell surface heparan sulfate proteoglycans with regulatory roles in cell adhesion, proliferation, and differentiation [Annu. Rev. Biochem. 68 (1999) 729]. While the syndecan heparan sulfate chains are essential for matrix binding, less is known about the signaling role of their core proteins. To mimic syndecan-specific adhesion, MDA-MB-231 mammary carcinoma cells were plated on antibodies against syndecan-4 or syndecan-1. While cells adherent via syndecan-4 spread, cells adherent via syndecan-1 do not. However, cells adherent via syndecan-1 can be induced to spread by Mn(2+), suggesting that activation of a beta(1) or beta(3) integrin partner is required. Surprisingly, pretreatment of cells with a function-activating beta(1) antibody does not induce spreading, whereas function-blocking beta(1) integrin antibodies do, suggesting involvement of a beta(1)-to-beta(3) integrin cross-talk. Indeed, blockade of beta(1) integrin activation induces alpha(v)beta(3) integrin activation detectable by soluble fibrinogen binding. Spreading in response to syndecan-1 is independent of integrin-ligand binding. Furthermore, competition with soluble murine syndecan-1 ectodomain, which does not disrupt cell adhesion, nonetheless blocks the spreading mechanism. These data suggest that the ectodomain of the syndecan-1 core protein directly participates in the formation of a signaling complex that signals in cooperation with alpha(v)beta(3) integrins; signaling via this complex is negatively regulated by beta(1) integrins.     

PDZ tandem of human syntenin: crystal structure and functional properties

Syntenin, a 33 kDa protein, interacts with several cell membrane receptors and with merlin, the product of the causal gene for neurofibromatosis type II. We report a crystal structure of the functional fragment of human syntenin containing two canonical PDZ domains, as well as binding studies for full-length syntenin, the PDZ tandem, and isolated PDZ domains. We show that the functional properties of syntenin are a result of independent interactions with target peptides, and that each domain is able to bind peptides belonging to two different classes: PDZ1 binds peptides from classes I and III, while PDZ2 interacts with classes I and II. The independent binding of merlin by PDZ1 and syndecan-4 by PDZ2 provides direct evidence for the coupling of syndecan-mediated signaling to actin regulation by merlin.     

The role of syndecans in disease and wound healing.

Syndecans are a family of transmembrane heparan sulfate proteoglycans widely expressed in both developing and adult tissues. Until recently, their role in pathogenesis was largely unexplored. In this review, we discuss the reported involvement of syndecans in human cancers, infectious diseases, obesity, wound healing and angiogenesis. In some cancers, syndecan expression has been shown to regulate tumor cell function (e.g. proliferation, adhesion, and motility) and serve as a prognostic marker for tumor progression and patient survival. The ectodomains and heparan sulfate glycosaminoglycan chains of syndecans can also act as receptors/co-receptors for some bacterial and viral pathogens, mediating infection. In addition, syndecans bind to obesity-related factors and regulate their signaling, in turn modulating food consumption and weight balance. In vivo animal models of tissue injury and in vitro data also implicate syndecans in processes necessary for wound healing, including fibroblast and endothelial proliferation, cell motility, angiogenesis, and extracellular matrix organization. These new insights into the involvement of syndecans in disease and tissue repair coupled with the emergence of syndecan-specific molecular tools may lead to novel therapies for a variety of human diseases.